Moving Picture Experts Group (MPEG) and Video Coding Experts Group (VCEG) have developed an improved and excellent video compression technology over existing MPEG-4 Part 2 and H.263 standards. The new standard is called H.264/AVC (Advanced Video Coding) and was released simultaneously as MPEG-4 Part 10 AVC and ITU-T Recommendation H.264. Such H.264/AVC (hereinafter referred to as ‘H.264’) uses a spatial predictive coding method, which is different from conventional video coding international standards such as MPEG-1, MPEG-2, MPEG-4 Part2 Visual and the like.
Conventional video coding methods use “intra prediction” for coefficients transformed in Discrete Cosine Transform Domain (or DCT Transform Domain) to seek higher encoding efficiency resulting in degradation of the subjective video quality at low band transmission bit rates. However, H.264 adopts the method of encoding based on a spatial intra prediction in a spatial domain rather than in a transform domain.
An encoder that uses a coding method based on the conventional spatial intra prediction predicts current block information from information of the previously encoded and reconstructed previous blocks, encodes information on just the difference of the prediction block from the actual block to encode, and transmits the encoded information to a decoder. Then, the encoder may transmit parameters needed for prediction of the block to the decoder or the encoder and decoder may be synchronized, so that they share the needed parameters for the decoder to predict the block. In terms of the decoder, the block information to be currently decoded is predicted using previously decoded and reconstructed adjacent block information and then added to the difference information transmitted from the encoder, which reconstructs the block to be decoded. Then, again, if the parameters needed for the prediction are transmitted from the decoder, the parameters can be decoded and used for prediction.
The above described intra prediction may be an intra_4×4 prediction, intra_16×16 prediction, intra_8×8 prediction and the like, where the respective intra predictions include a plurality of prediction modes.
FIG. 1 is a diagram showing conventional nine 4×4 intra prediction modes.
Referring to FIG. 1, the intra_4×4 prediction has nine prediction modes which include a vertical mode, horizontal mode, direct current (DC) mode, diagonal down-left mode, diagonal down-right mode, vertical-right mode, horizontal-down mode, vertical-left mode and horizontal-up mode.
FIG. 2 is a diagram showing conventional four 16×16 intra prediction modes.
In FIG. 2, the intra_16×16 prediction has four prediction modes which include a vertical mode, horizontal mode, DC mode and plane mode. The intra_8×8 prediction also has four modes similar to the intra_16×16 prediction.
All of the intra prediction modes described in FIGS. 1 and 2 predict the current blocks by generating predicted pixel values from the previously encoded or decoded pixels neighboring the current blocks. Typically in H.264, the intra predictions are performed in square block unit, and the prediction blocks for the intra prediction are sized 16×16, 8×8, and 4×4 for the intra_16×16 prediction, intra_8×8 prediction, and intra_4×4 prediction, respectively.
The reason for carrying out intra predictions in the N×N sized square is that the subsequent discrete cosine transform (hereinafter called DCT transform) for compressing the post-intra prediction residual signals and the quantization procedure are likewise performed by regular square (4×4, 8×8). However, performing the intra predictions by such regular square blocks may degrade the accuracy of prediction since the pixels to be predicted are predicted using relatively distant pixels, resulting in low efficiency of the compression.
FIG. 3 illustrates adjacent pixels and the current block pixels used for the typical intra_16×16 prediction.
256 pixels in the lower case letters a0 to p15 represent the pixels in the current block, while 32 pixels in the upper case A to AF are adjacent pixels of the neighboring blocks which had been compressed prior to the compression of the current block. For the vertical intra prediction in FIG. 3, the first column of pixels a0˜a15 may be predicted from pixel A, and the second column of pixels b0˜b15 may be predicted from pixel B. The remaining pixels c0˜p15 may be predicted from the upper pixels C˜P, respectively. In this case, the pixels a0˜p0 are predicted from the adjacent pixels A˜P with the spatially closest proximity in the direction of prediction, but the pixels a1˜p1 are spatially distanced from the adjacent pixels A˜P by two pixels as the pixels a15˜p15 are spatially distanced from the adjacent pixels A˜P by sixteen pixels, which degrades the accuracy of the intra prediction and in turn the compression efficiency.